Correct kerning of hyperlinks in math mode

Question

I use hyperlinks in math mode to if a reader does not read my thesis sequentially and finds a very arcane symbol somewhere, he may just click it and go to where it is defined. However, adding links changes the kerning making subscripts and superscripts more spaced out and other ugly things.

Consider this MWE:

\documentclass{article}

\usepackage{amsmath, amssymb, fontspec, unicode-math, tikz}
\usepackage[colorlinks=true, linkcolor=black]{hyperref}

\begin{document}

Let \hypertarget{a}{$X\colon[0,1]\times\Omega\to\mathbb{R}$} be a stochastic
process.

\tikz[overlay, baseline, anchor=base west, inner sep=0, blue]
  \node {${X}_t$};%
\tikz[overlay, baseline, anchor=base west, inner sep=0, red, opacity=0.5]
  \node {$\hyperlink{a}{X}_t$};

\end{document}

The output is

I'm using LuaLaTeX and texlive 2013. Is there any way to correct the kerning?

Answer 1

Edit A real solution uses this answer:

\documentclass{article}
\usepackage{amsmath, amssymb, tikz}
\usepackage[colorlinks=true, linkcolor=black]{hyperref}

\newcommand\hypersub[2]{\hyperlink{#1}{\vphantom{#2}\smash{#2_{}\kern-\scriptspace}}}

\begin{document}
\tikz[overlay, baseline, anchor=base west, inner sep=0, blue]
  \node {${X}_t$};%
\tikz[overlay, baseline, anchor=base west, inner sep=0, red, opacity=0.5]
  \node {$\hypersub{a}{X}_t$};
\end{document}

end edit

This is only a workaround for use when you want to link the primary term excluding the subscript. Defining a specific hyperlink for when a subscript follows as:

\newcommand\hypersub[2]{\hyperlink{#1}{#2}\hspace{-.08em}} 

allows:

\tikz[overlay, baseline, anchor=base west, inner sep=0, blue]
  \node {${X}_t$};%
\tikz[overlay, baseline, anchor=base west, inner sep=0, red, opacity=0.5]
  \node {$\hypersub{a}{X}_t$};

to produce:

The mismatch is around 2% of the stroke width of the t.

Answer 2

The explanation is the the text of a hyperlink has to be surrounded by \special commands. These are technically called "whatsits". These essentially force TeX to wrap up its processing of the text. In math mode, this includes appending an italic correction. Since the hyperlink text is a math italic X, the italic correction is nonzero. You get the same shifting of the subscript if you put any other whatsit between The X and its subscript. Examples are \write commands and color changes. Try, for example, $X\write16{test}_t$ or $\color{red}X\color{black}_t.